I straightened the edge with a break away razor. It's mostly ok now, but as I had hoped not to do I have unequal front edge overlap on the outside corner to the wall. 1" down to 9/16". The correct thing was to scribe the board to the wall. But what has happened is why I did not want to.

I'm starting to feel like this is the unlevel table issue.
Take an inch off each table leg its still unlevel. Keep going eventually it's only 1 foot high.

I'm looking to work more on this project. Got the plumbing done; moved the shutoff valves down below the vanity, but the plumber had to notch out a stud to make room to get a couple of fingers in to access it. The reason being the cold water line to the toilet crosses behind the hot at 90 degrees and leaves no room to work. And no room to anchor wallboard to make a tidy access panel. I jury rigged a sheetrock panel with hopes not to need to shut off any time soon.

Back to the countertop;

I got the kerdiboard edges trimmed to match the wall issues. I also now have only 1/4" of kerdiboard overlapping the vanity edge due to the trimming. But I can live with it since I have to. (Can I glue kediboard edge to edge with kerdifix at all? I have intact cut pieces.)

I still have a bow in the front edge of the vanity or warped Kerdiboard.

After a couple of go arounds with shims I tried something low tech.
I put a 50 pound sack of versabond on top. The gap disappears on
all sides.

So my question now is about shear strength of kerdifix and the kerdi on the kerdiboard. If I put kerdifix on the edges of the vanity, position the board as I want it, and put the weight on top will the kerdifix hold?

OR will the shearing force of the bow eventually break the kerdiboard loose?
Remembering too I still must cut a hole for the sink cutout which could jar something loose.

I got it figured out how to fix the gap. After months of trying shims
that were not going to work.

I found short screws and put a piece of composite wood (same type I was told to use as a ledger board in the shower) on the inside of the front edge of the cabinet. I kept thinking they would poke through the laminate ruing it.
1.5" screws did the trick though.

I was able to use the level as a guide and it seems to be working OK with kerdiboard on top. It's now flush with the top of the vanity edges on rear, left & right sides and with this piece of wood in front albeit the ends still have some gap. I cannot fix the gaps since they are over the two drawers.
It's more than enough to make sure once fixed in place it's not going to move or come loose -ever- without wrecking the kerdiboard.

I have a tiny bit of unlevel toward the wall too.
But it's not an issue unless someone tells me otherwise.

I got my neighbor to use his rock-steady hand to xacto blade the sink
template. Now I just need to cut out the sink hole. I was waiting for
the kerdifix to dry fully to be sure it's not gonna jiggle loose while
cutting out the hole. I marked the hole position.

The only thing that can go wrong now is screwing up the cut for the
self rimming sink. I've done this before but never in kediboard.

FWIW, in class, they told us that KerdiFix cures at about 1/8"/day from each edge towards the middle. So, depending on how it's applied, it could take awhile to gain full strength. But, the cured stuff is very strong. ON a 2" thick piece, triangular shape, Kerdifixed into a corner to Kerdiboard from around 4pm to 9am, I was able to stand on it and it held my 220# or so. Considering that only about barely 1/8th inch out of the 2" thick thin layer had cured in that timeframe, I was impressed.

Yeah the bond strength is impressive. I've seen the Schluter videos of putting 3 men on a shower seat made of kerdi board with no bottom support. I cannot remember if that was kerdifix onto the wall or not. I only used a tiny bead (the new kerdi fix has a pre sized hole in the tip which I presume IS 1/8" or less) except on the four corners where I glopped it more.

BTW, how air tight is the tip with that screw-on cap? Do I need to put it into a zip lock? I don't want it to dry out before I get to the master bath countertop in a week or so. (might be too late )

I got the sink hole cut *finally*. I'm terrible at making such cuts. Almost ended up with an oval shape instead of round (I must have twisted the jigsaw while cutting even though I thought it was flat) so I'm glad this is a top mount self rimming sink. I may need to be creative tiling that area.

I have another bathroom to do after this one with kerdiboard so I will have to make sure I am more careful in making *that* cut.